1. Field of the Invention
The present invention relates to a light emitting diode, and particularly to a light emitting diode having a structure capable of heightening the optical output and a method for manufacturing the same.
2. Description of Related Art
A light emitting diode array in which planar-type light emitting diodes (LEDs) are arranged in an array has been put to practical use for an LED printer and the like. An example of the fundamental structure of such a former light emitting diode array is briefly described with reference to FIG. 7 and FIGS. 8(A) to 8(C). The light emitting diode array 10 is provided with an insulating film 16 on one main surface (upside) 12a of an n-type semiconductor region 12. The insulating film 16 has a plurality of diffusion windows 20 formed by etching. These diffusion windows 20 are arranged at regular intervals in a line. This array 10 has individual p-type diffusion regions, namely, p-type semiconductor regions 28 each of which is formed by diffusing zinc (Zn) as an impurity into an n-type semiconductor region 12 exposed in a diffusion window 20. The p-type semiconductor region 28 is an island-shaped surface-diffused region surrounded by the n-type semiconductor region 12, and the respective upsides 12a and 28a of the n-type and p-type semiconductor regions are in a common plane. The junction 30 between both the regions is in the shape of a dish. Ordinarily, the p-type semiconductor regions adjacent to each other are isolated from each other. Individual p-side electrodes 24 electrically connected to the respective p-type semiconductor regions are formed on the insulating film 16. An n-side electrode 26 is formed on the other main surface (reverse side) of the insulating film 16.
FIG. 8 shows part of the former light emitting diode array, focusing on one LED. Particularly, FIG. 8(A) is a sectional view taken along line X-Y of FIG. 7. FIG. 8(B) is a plan view mainly showing the p-type semiconductor region exposed in the diffusion window 20. FIG. 8(C) is an optical output characteristic curve diagram for explaining an output characteristic of this LED, and the abscissa shows a position and the ordinate shows an optical output (in an arbitrary unit).
The p-type semiconductor region 28 described above comprises a first partial region R1 which is constant in depth (thickness) from its upper surface 28a and a second partial region (also referred to as a peripheral region) R2 which is its peripheral region and is shallower (thinner) than the first partial region R1 in depth (thickness) from the upper surface 28a. Since the junction 30a of the first partial region R1 in the junction interface is substantially in parallel with the upper surface, the depth of the junction has a constant value L1. On the other hand, the junction 30b of the second partial region R2 becomes gradually shallower as it becomes more distant from the first partial region R1. Finally, the junction 30b ends at the boundary between the upper surfaces 12a and 28a (the peripheral edge of the p-type semiconductor region 28) R20.
Therefore, the thickness of the second partial region R2 gradually varies from depth L1 to depth xe2x80x9c0xe2x80x9d according to a position in it.
When letting an electric current flow between both the electrodes 24 and 26 of an LED 10 having such a structure as this, electrons and holes are recombined in the junction 30 to generate light. Generated light B passes through the p-type semiconductor region 28 and is outputted from the diffusion window 20 (see FIG. 7).
Hereupon, the light B1 generated at the junction 30a of the first partial region R1 passes through said first partial region R1 being thicker in thickness and then is outputted. The light B2 generated at the junction 30b of the second partial region R2 passes through said second partial region R2 being thinner in thickness and then is outputted. Now, it is assumed that a quantity of light generated in a unit area of the junction interface is constant. The generated light passes through these first and second partial regions R1 and R2 in the direction perpendicular to the upper surface of these regions. In this case, a quantity of light absorbed in these partial regions is the maximum in the first partial region R1. A quantity of light absorbed in the second partial region R2 is the maximum at the boundary between the first and second partial regions and is xe2x80x9c0xe2x80x9d at the peripheral edge R20 of the second partial region R2. It becomes gradually smaller as being closer to the peripheral edge R20 in the intermediate portion of the second partial region.
It is therefore known that the power of light outputted from the upper surface 28a of the p-type semiconductor region 28 is the minimum I0 in the first partial region R1, and becomes gradually larger so that it is the maximum I0+i ( greater than I0) at the peripheral edge R20 in the second partial region R2 (see FIG. 8(C)).
In a former LED having a structure like this, as described above, since light is absorbed in the p-type semiconductor region, the total optical power of outputted light is made smaller. Thereupon, up to now a desired large optical power has been obtained by applying a high voltage between the n-side and p-side electrodes 24 and 26, but applying a high voltage as described above has caused a problem that the power consumption becomes high.
As a result of various attempts at solving this problem, the inventors have found that if part of the junction of the second partial region, which has been up to now formed so as to be constant in depth from the upper surface, is formed as a shallower junction, absorption of light can be reduced corresponding to the depth becoming shallower, and thereby have attained the present invention.
Thus, an object of the present invention is to provide a light emitting diode capable of outputting a high-power light without applying a high voltage between the electrodes.
Another object of the invention is to provide a method for manufacturing such a light emitting diode.
In order to attain the objects, according to a first aspect of the present invention, there is provided a light emitting diode (LED) provided with such a structure as described below. This LED is provided with a first conductive-type semiconductor region and a second conductive-type semiconductor region which is buried in the first conductive-type semiconductor region and forms a junction with the first conductive-type semiconductor region. The junction at the bottom of the second conductive-type semiconductor region (said junction being here referred to as a bottom junction) varies in depth from the surface of the second conductive-type semiconductor region according to a position in it.
According to such a structure, when making the maximum depth of the bottom junction of the second conductive-type semiconductor region coincide with the depth of the bottom junction of a former LED, a junction depth at another position in the bottom is shallower than this maximum depth. Therefore, the bottom junction includes a deep junction and a shallow junction. This means that the second conductive-type semiconductor region includes a deep portion and a shallow portion, in other words, that a place where generated light is more absorbed and a place where generated light is less absorbed. Absorption of light depends upon the thickness of the second conductive-type semiconductor region which the light passes through. Accordingly, even if an electric current of high density does not flow between the electrodes by applying between them a high voltage. By this, it is possible to heighten a quantity of output light, namely, an optical power of the LED corresponding to a reduction in absorption of light in the second conductive-type semiconductor region.
In implementing the present invention, the second conductive-type semiconductor region preferably forms a junction with the first conductive-type semiconductor region by having its bottom face and peripheral side face surrounded by the first conductive-type semiconductor region. The upper surfaces of the first and second conductive-type semiconductor regions are in the same plane. The upper surface of the second conductive-type semiconductor region forms a light outputting surface. This second conductive-type semiconductor region comprises a first partial region and a second partial region. The first partial region is a region which is put between the central area of the upper surface of the second conductive-type semiconductor region and the junction at the bottom. The second partial region is a peripheral region which is put between the peripheral area of the upper surface of the second conductive-type semiconductor region and the junction at the peripheral side face and is in contact with the first partial region. This second partial region becomes gradually shallower in depth (namely, thickness) from the upper surface of it as coming from the first partial region nearer to the peripheral edge. That is to say, the depth of the junction at the peripheral side face (said junction being here referred to as a side junction) gradually varies from the depth of the bottom junction to depth xe2x80x9c0xe2x80x9d.
In a structure like this, it is preferable that the first partial region is composed of a plurality of subregions being different in depth (namely, thickness) from one another and the maximum depth of these subregions is substantially equal to the maximum depth of the second partial region.